This invention relates to a motor control system for precisely regulating the torque of an induction motor, and more particularly to a motor control system capable of maintaining constant torque at a selected value independent of machine parameter changes.
Several induction motor closed loop control strategies are available that produce constant torque independent of speed. Most of these strategies are based on the technique of holding slip frequency constant while maintaining constant volts per hertz with stator resistance voltage drop compensation. One control configuration for maintaining constant torque is to maintain constant slip frequency and constant stator current magnitude, while another maintains constant slip frequency and real component of stator current with respect to the terminal voltage compensated for the drop across the stator resistance. These control strategies are based on the assumption that all machine parameters remain constant independent of time and operating point. In reality, the motor heats up changing most parameters with the motor resistance being the most notable. Since rotor temperature is both difficult to measure and to predict in an actual drive, these controls cannot effectively regulate torque. Ignoring magnetizing losses, the developed torque can only be regulated by controlling the total power which crosses the airgap and is dissipated by the rotor resistance, and approximations used in the past based on measuring the input power and subtracting off stator copper losses are not sufficient.